1. Technical Field
This invention relates to a nitriding furnace apparatus which is used for forming a nitrided layer on the surface of steel material.
2. Prior Art
A technology for forming a nitrided layer on the surface of steel material is utilized widely from a respect of carrying out hardening the steel surface to improve characteristics such as weal resistance. Such nitriding is conducted as follows. A clean surface is exposed by pretreating to remove a passive surface coat layer such as an oxidized layer and in that state the clean surface is contacted with nitrogen source gas such as ammonia to penetrate and diffuse inside the steel. Generally the pretreatment to the surface of the steel material, in particular the steel material containing a large amount of Cr, is carried out by cleaning the steel surface with a hydrofluoric acid-nitric acid mixture.
However, it is difficult to remove the stubborn passive coat layer on the surface of stainless steel, particularly austinitic stainless steel, even with the cleaning with the hydrofluoric acid-nitric acid mixture, and even if the passive coat layer is removed, it is likely to be reformed before reaching nitriding temperature. For this reason, it is practically impossible to form a nitrided layer with sufficient thickness on the steel surface uniformly due to the remaining passive coat layer in a conventional nitriding. Improvement has been required.
The present invention recognizes that the pretreating prior to nitriding influences a state of the nitridine largely. As a result, it was found to be quite effective to hold the steel material in the atmosphere of fluorinated gas using the fluorinated gas containing at least one fluorine source gas selected from NF.sub.3, BF.sub.3, CF.sub.4, HF, SF.sub.6 & F.sub.2 in an inert gas such as N.sub.2. That is, when the steel material is held in said atmosphere in a heated state, a passive coat layer on the steel surface turns into a fluorinated layer by action of an active F atoms of said fluorinated gas. The fluorinated layer is decomposed by H.sub.2, NH.sub.3 or a small amount of water to expose the steel surface in a bare state. Since the bare state metallic surface is cleaned and activated, it is easy for N atoms to penetrate/diffuse from the steel surface to the inside thereof when nitriding. The inventors have filed a patent application based on this concept entitled "A method of nitriding steel", as Japanese patent Application No. 1-177660. The method thereof is carried out by using a heat treatment furnace of which the inside comprises one chamber as shown in FIG. 3. That is, the steel material (not shown) put in a metallic container 2 is charged into said furnace 1 and heated at the temperature of about 300.degree. C..about.400.degree. C. by a heater 3. And the steel material is pretreated by introducing fluorinated gas, in which NF.sub.3 is contained in N.sub.2 gas, into the furnace 1 through gas inlet pipe 4. Then, after finishing the pretreatment, said fluorinated gas is taken out through a gas exhaust pipe 5 and released to outside, subsequently the heater 3 is electrically loaded to raise the temperature of the steel material to 400.degree. C..about.600.degree. C. In that state, mixed gas (e.g. NH.sub.3 : 50%, CO.sub.2 : 10%, CO: a small amount, H.sub.2 : a small amount, N.sub.2 : rest) is introduced to the furnace 1 through said pipe 4 to nitride the steel material. In this case, a fluorinated layer formed on the steel surface with H.sub.2, NH.sub.3 and the like in said mixed gas is destroyed to expose the metal surface, N atoms from NH.sub.3 acts against the exposed activated metal surface to form a nitrided layer deeply and uniformly on the steel surface. However, in the heat treatment furnace 1 with this structure, since said pretreatment and nitriding are conducted in one furnace, the following problems arise. That is, in said pretreatment, fluorinated gas is introduced into said furnace 1. NF.sub.3 which is an effective ingredient in the fluorinated gas acts not only against the steel surface but also against inner wall surfaces of the heat treatment furnace 1 to form a fluorinated layer thereto. The formed fluorinated layer is decomposed and removed when subsequent nitriding as well as that on the steel material surface. Therefore, NF.sub.3 used for covering the inner wall surface of the heat treatment furnace 1 is uneconomical. The fluorinated layer thus decomposed and removed from the inner wall of the furnace 1 reacts on ammonia used in nitriding to be NH.sub.4 F finally and it is exhausted to outside. Not only the fluorinated layer on the steel surface but also that on the inner wall of the furnace 1 are turned into NH.sub.4 F to be exhausted. Thereby, there is a problem that an exhaust pipe 5 of the heat treatment furnace 1 is easily filled with NH.sub.4 F too much and stopped up because the produced amount of NH.sub.4 F is too large. Furthermore, it is necessary to cool the nitrided steel in the furnace 1 after said nitriding, but there is another problem in that since the whole furnace is heated by the heat for nitriding, temperature of the steel material does not go down easily and it takes more than 4 hours for cooling it. In FIG. 3, the reference numeral 6 indicates an adiabatic wall, the numeral 7 an opening and closing door, 8 fans, 9 a frame, 10 a column for a frame, 11 a column of furnace body, 12 a vacuum pump, and 13 an exhaust gas treatment apparatus.